Conventional electron beam emitting filaments are produced in several geometric configurations as shown in FIGS. 1A and 1B. The conventional electron beam emitting filament depicted in FIG. 1A comprises arcuate legs 10 terminating at tip 11 and affixed to posts 12 which, in turn, are mounted in base 13. The conventional electron beam emitting filament depicted in FIG. 1B comprises legs 14 forming an acute angle at tip 15 and affixed to posts 16 which, in turn, are mounted in base 17.
Another conventional electron beam emitting filament configuration, which is shown in FIG. 2, comprises a filament 20 forming a loop tip 21. The ends of filament 20 are affixed to base 22. An enlarged tip of loop 21 is shown in FIG. 3, wherein the arrows 30 represent emitted electrons. Electron beam emitting filaments comprising a loop exhibit superior mechanical stability and longevity.
Conventional electron beam emitting filaments are typically produced from tungsten (W) or lanthanum boron (LaB.sub.6) and operate by direct electron transmission into free space. Such electron beam emitting filaments enjoy utility and various applications, such as cathode ray tubes, field emission guns and particularly in electron microscopes.
An important characteristic of electron beam emitting filaments is the ability to generate a high degree of brightness and contrast while maintaining mechanical stability for a long operating life. The contrast and brightness of an electron beam emitting filament, as well as its mechanical stability, are dependent upon, inter alia, the geometric configuration of the tip from which the electrons are emitted.
In accordance with conventional practices, electron beam emitting tips are typically fashioned by etching, e.g., chemical or electrochemical etching. Electron beam emitting filaments, including loop filaments, produced in accordance with conventional techniques have tips with a radius of curvature between about 1000 .ANG. and about 5000 .ANG.. Typical electron beam emitting filaments are commercially available from Energy Beam Sciences, Inc., Agawam, Mass. There exists, however, a need for electron beam emitting filaments, particular for use in electron microscopes, which provide a brighter source of electrons than attainable with conventional electron beam emitting filaments, particularly conventional electron beam emitting filaments made from tungsten.
A probe microscope operates in a manner quite unlike an electron microscope. For example, a probe microscope comprises a probe, typically of polycrystalline tungsten and/or iridium, which basically functions as a stylus by tracing various topographies. The geometrical configuration of the tips of such probes is designed to provide access to various topographies which are encountered so that the scan line can actually reflect the depth and the width of a trench as well as the angle of the sidewalls characteristic of the topography undergoing scanning. It has recently been reported that the focused ion beam (FIB) milling has been used to provide a desirable geometric configuration for the tip of a probe for use in a probe microscope. See L. C. Hopkins et al., "Polycrystalline Tungsten and iridium probe tip preparation with a Ga.sup.+ focused ion beam," Journal of Vacuum Science Technology B 13(2), March/April 1995, pp. 335-337. Hopkins et al. disclose that FIB milling was employed to produce a probe having a tip with radius of curvature of 5 to 10 nm during a second milling step subsequent to etching. The resulting conical shape, which permits extension of profilometry to high-aspect-ratio features, was considered to be unexpected based upon theoretical calculations. It was further reported that the mechanism that produces the microtips is not known and that further experimentation is warranted.